Sonic water level measuring method and system there for

ABSTRACT

A sonic water level measuring method and system for mounting a wave-guide tube for measuring a water level in a reservoir, a river, etc. generating sonic pulses at the upper of the wave-guide tube, measuring transit times that it takes for the sonic pulses to transit them through air medium and transit back to an original point after being reflected on the water surface, and multiplying the transit times by a sound velocity to compute a water level comprises steps of measuring the sound velocity using the times that it takes for the sonic pulse to transit in two intervals in order to exactly measure the sound velocity that is changed according to the changing of an air temperature, pressure and component in the wave-guide tube; and measuring the water level considering an arithmetical average value of the sound velocities measured as a sound velocity in the wave-guide tube.

BACKGROUND OF THE INVENTION

[0001] The invention is related to a technology for measuring a water level using a sonic wave, and particularly, to a method for exactly measuring a water level using a sonic wave independent of a measuring range in a reservoir, a underground water and a larger river, etc., and system therefore.

PRIOR ART

[0002] A measuring range of a reservoir, a river and a underground water may be larger, but the hydrology observation requires not to exceed the allowance error ±5˜10 mm throughout a full range of a water level to be measured.

[0003] Typical sonic level measuring methods and/or systems therefore for satisfying such conditions are disclosed as follows:

[0004] U.S. Pat. No. 5,842,373 issued on Dec. 1, 1998

[0005] Germany Patent No. 19511234 published on Sep. 11, 1997

[0006] Japanese Patent No. 2,756,647 issued on Mar. 13, 1998

[0007] Korean Patent No. 250,714 issued on Jun. 16, 1998

[0008] The conventional sonic water level measuring method will be described with reference to FIG. 1. 1 is a sound generator, 2 is a wave-guide tube and 5 ₁, 5 ₂, 5 ₃ . . . 5 _(n) are a sound receiver.

[0009] The first sound receiver 5 ₁ is placed on an original position 0, and a distance L to a water surface there from is measured.

[0010] The sound generator 1 is operated to transit a sonic pulse toward a water surface along the wage-guide tube 2. Then, the sonic pulse is reflected on the water surface and transited upward. At that time, a time interval t₁ from a moment that the sound receiver 5 ₁ receives an incident time until it receives the sonic pulse reflected on the water surface is as follows: The measuring error of t₁ is ignored. $\begin{matrix} {t_{1} = \frac{2L}{C_{1}}} & (1) \end{matrix}$

[0011] Wherein, C₁ is a sonic velocity in an interval L.

[0012] Similarly, a time interval t₂ from a moment that the sound receiver 5 ₁ receives an incident wave until the sound receiver 5 _(n) receives the incident wave transited thereto is as follows: $\begin{matrix} {t_{2} = {\frac{L_{0}}{C_{2}} = \frac{\left( {h - 1} \right)l}{C_{2}}}} & (2) \end{matrix}$

[0013] Wherein, C₂ is a sonic velocity in an interval L₀, n is the number of the sound receiver and t is an interval between the sound receivers 5 _(i) and 5 _(i+1).

[0014] Therefore, L is obtained from the expressions (1) and (2) as follows: $\begin{matrix} {L^{\prime} = {{\frac{t_{1}}{2t_{2}} \cdot L_{0}} = {\frac{t_{1}}{2t_{2}}\left( {n - 1} \right)l}}} & (3) \end{matrix}$

[0015] But, the exact value of L is as follows: $\begin{matrix} {L = {\frac{t_{1}}{2t_{1}}L_{0}\frac{C_{2}}{C_{1}}}} & (4) \end{matrix}$

[0016] Wherein, the expression (3) is established under the assumption that C₁=C₂.

[0017] In summer, a temperature in the upper portion of the wave-guide tube is higher than that in the lower portion of the wave-guide tube. On the contrary, when an atmosphere air temperature is lower than an air one, the air temperature in the upper portion of the wave-guide tube becomes lower than that in the lower portion of the wave-guide tube. Therefore, for C₁=C₂, L≈L₀ must be established. In other words, the interval l between the sound receivers is selected to become as small as possible, and an interval ΔL=L_(i)−L_(0i) of the sound receiver disposed closest to the water surface in the wave-guide tube is selected. In the conventional sonic water level measuring system, the value l is selected as follows: $\begin{matrix} {l \leq {{\Delta_{L}C_{0}} + \frac{0.5_{\alpha}\left( {T_{0} + T_{w}} \right)}{0.5_{\alpha}\left( {T_{0} - T_{w}} \right)}}} & (5) \end{matrix}$

[0018] Wherein, T₀ is an air temperature in the position of the sound receiver 5 ₁; T_(w) is an air temperature on the water surface of the wave-guide tube; C₀ is a sonic velocity of 331.6 m/s, when T=0° C.; is a temperature coefficient, in which ≈0.6 and Δ_(L) is an allowance error of the L measurement.

[0019] The expression (5) is derived under the assumption that the air temperature is changed with a constant gradient $\frac{T_{0} - T_{L}}{L}$

[0020] of a straight line (referring to FIG. 5).

[0021] When T₀=40° C., T_(w)=25° C. and Δ_(L)=0.01 m(1 cm), l is as follows:

l≦0.78 m

[0022] if the water level is changed in the range of 20 m, the number of the receiver 5 _(i) is as follows: ${n \geq \frac{20}{0.78}} = {25.6 \approx 26}$

[0023] If the allowance error of the water level measurement Δ_(L)=±5 mm, N=52. In other words, a large number of the sound receivers are required.

[0024] A conventional technical feature is that the interval between the sound receivers is narrowed, the number of the sound receiver is increased and the accuracy of the water level measurement is enhanced

[0025] But, the advantages are as follows:

[0026] The increasing of the number n of the sound receiver causes the water level meter to become complex and thus results in heightening the failure possibility of the sound receiver. For example, the sound receivers are positioned below or over the water surface according to the water level change. If the water level is raised, the sound receiver disposed on the lowest portion of the system is under a larger water pressure. Against this situation, the sound receivers are thoroughly waterproofed and their receiving sensitivities are kept at a uniform state in air as well as in water. And, the sound receivers are made into a compact size and free of the failure. Due to these, the sound receiver is complex and expensive. It is found out from much experience that the failure ratio of the sound receiver is highest among parts of the system.

[0027] A next problem exists in being not able to narrow the interval l between the sound receivers, auxiliary. The reason is as follows:

[0028] In order to secure the higher water level measuring range, a sonic pulse of a lower frequency is used. As shown in FIG. 2, it takes much time until the sonic pulse is fully attenuated. In FIG. 2, a dotted line is a reflected pulse. If f=1000 Hz, and the pulse requires 6 periods for the fully attenuation, $\tau = {\frac{6}{1000} = {6 \times 10^{- 3}{S.}}}$

[0029] A transit time t_(ΔL) that the sonic pulse is reflected on the water surface and transited to the sound receiver closet to the water surface is as follows: $t_{\Delta \quad L} = \frac{2\Delta \quad L}{C_{\Delta \quad L}}$

[0030] In order to get the sound receiver to receive an incident wave and then reflected wave, exactly, t_(ΔL)≧τ. If C_(ΔL)=348 m/s, ΔL is as follows: ${\Delta \quad L} = {\frac{\tau \cdot C_{\Delta \quad L}}{2} = {\frac{6 \cdot 10^{- 3} \cdot 348}{2} = {1.04m}}}$

[0031] It means that l can't be selected below 1.04 m. In order to select a small value of l, the frequency of the sonic pulse must become larger. As the frequency of the sonic pulse is increased, the damping becomes larger. For it, the water level measurement can't be secured in a larger range. The water level measurement range is usually 50 m.

[0032] The patent discloses that l is selected to be equal to 0.78 m in order to secure the water level measuring error ΔL=±1 cm, but if the frequency of the sonic pulse f is equal to 1000 Hz, it is not possible to measure the water level. Even through f is selected to be 1.04 m, as the water level ΔL is raised a little, the corresponding sound receiver is switched into another sound receiver to be operated. As a result, if l is selected to be larger than 1.04 m, the water level measuring error is increased. It is now used to be l≈2 m for the manufacturing of the system.

[0033] The conventional technology has limitations in securing the accuracy of the water level measurement. Also, the accuracy of the water level measurement can't be secured even through the number of the sound receiver is increased l is decreased.

[0034] Accordingly, an object of the invention is to provide a sonic water level measuring for securing the higher accuracy in a wider water level measuring range with a number of sound receivers being not used and system there for.

SUMMARY OF THE INVENTION

[0035] A sonic water level measuring method and system there for comprises two sound generator mounted toward the water surface on the upper of a wave-guide tube, which are spaced away at an interval l₁ from each other; a float submerged in water and including a projector integrally extended there from with a diameter of 0.5 times as large as that of the wave-guide tube, the upper surface of which is kept at a height of l₂; and a sound receiver receiving incident waves upon the generating of a sonic pulse and then sonic waves reflected in order on the upper surface of the projector and the water. Therefore, a sonic water level measuring method comprises steps of measuring a sonic pulse transit time t_(L) from a moment that a first sound receiver receives an incident wave until the first sound receiver receives a wave reflected on the water surface, measuring times t₁ and t₂ that the sonic pulse is transited in an interval l₁ between two sound receivers and in an interval l₂ between the upper surface of the projector and the water surface and measuring a water level based on the measured values as follows: $L = {{\left( \frac{t_{L}}{2} \right) \cdot \frac{1}{2}}\left( {\frac{l_{1}}{t_{1}} + \frac{2l_{2}}{t_{2}}} \right)}$

[0036] Wherein, two sound receivers disposed on the upper of the wave-guide tube is always mounted to be in air, and the projector integrated with the float is called “Sound Reflecting Cylinder” below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention will be described in detail with reference to the accompanying drawings, in which:

[0038]FIG. 1 is a view illustrating a sonic water level measuring method;

[0039]FIG. 2 is a view illustrating a wave shape of a sonic pulse;

[0040]FIG. 3 is a view illustrating a sonic water level measuring method according to the invention;

[0041]FIGS. 4A to 4C are views illustrating wave shapes generated upon the receiving time of the sonic pulse according to the invention;

[0042]FIG. 5 is a view illustrating a distribution of an air temperature in a wave-guide tube;

[0043]FIG. 6 is a view illustrating a distribution of an air temperature in a wave-guide tube;

[0044]FIG. 7 is a block diagram illustrating a sonic water level measuring system according to the invention;

[0045]FIGS. 8A to 8D are views illustrating wave shapes obtained by adjusting an amplitude of an output signal upon the receiving of an incident wave and a reflecting wave according to the invention;

[0046]FIG. 9 is a view illustrating the mounting of a microphone constituted as a sonic water level measuring system according to the invention;

[0047]FIG. 10 is a view illustrating the configuration of a float according to the invention; and

[0048]FIG. 11 is a view illustrating the operating position of the float according to the invention, if the wave-guide tube is mounted on the slant.

DETAILED DESCRIPTION OF THE INVENTION

[0049] According to the invention, a sonic water level measuring method now will be explained in detail with reference to FIGS. 3 and 4.

[0050]FIG. 3 shows a part of a sonic water level measuring system for performing a sonic water level measuring method according to the invention.

[0051]1 is a sound generator, 2 is a wave-guide tube, 3 and 4 are sound receivers (called “microphone” below), 6 is a hollow sound reflecting cylinder and 7 is a float integrated with the hollow sound reflecting cylinder 6. 8 is a microphone rod for mounting the microphone 4 to adjust an interval l₁ between the microphone 3 and 4. l₂ not explained is an interval between the upper surface of the sound reflecting cylinder 6 and the water surface.

[0052] The position of the microphone 3 is subject to be an original point of the water level measurement. Assuming that the original point has a height H₀ above sea level, a water level H of a reservoir or open sluice is as follows:

H=H ₀ −L

[0053] L is a distance from the position of the microphone 3 to the water surface. The distance L is measured using a sonic wave as follows:

[0054] The sound generator 1 generates sonic pulses. The sonic pulse is transited toward the water surface along the wave-guide tube 2, which is defined into an incident wave. The incident wave is first reflected on the upper surface of the sound-reflecting cylinder 6 and on the water surface in order, and then transited upward along the wave-guide tube, which is called “reflected wave”. At that time, the microphones 3 and 4 receive the incident waves in order, and then the reflected waves reach in an order of the microphones 4 and 3.

[0055]FIG. 4A shows wave shapes of the outputting signals that the microphones 3 and 4 generate in times, and FIGS. 4B and 4C show wave shapes of a single pulse signal in times to catch moments that the outputting signals of the microphone 3 and 4 are amplified and a zero crossing circuit generates a zero potential signal at one period of the amplified signals. (3) and (4) shown in FIG. 4A are the outputting signal of each of the microphone 3 and 4 (I), (II), (III) . . . (VI) shown in FIGS. 4B and 4C are a sequence number of a pulse in times that the outputting signals of the microphone 3 and 4 are caught. A time interval between pulses (I) and (II) or pulses (V) and (VI) is a time t₁ that it takes for the sonic pulse to transit the interval l₁ between the microphones 3 and 4. A time interval between pulses (IV) and (VI) or pulses (III) and (V) is a time t₂ that it takes for the sonic pulse to transit a distance two times as long as an interval l₂ between the upper surface of the sound reflecting cylinder 6 and the water surface. A time interval between pulses (I) and (II) is a time t_(L) that it takes for the sonic pulse to transit to and from a distance L between the microphone 3 and the water surface.

[0056] For example, $\begin{matrix} {t_{1} = \frac{l_{1}}{C_{1}}} & (7) \\ {t_{2} = \frac{2l_{2}}{C_{2}}} & (8) \\ {t_{2} = \frac{2L}{C_{L}}} & (9) \end{matrix}$

[0057] Wherein, C₁, C₂ and C_(L) are a sound velocity of interval l₁, l₂ and L.

[0058] On the other hand, the microphone 3 can be omitted. Instead of it, a bottom surface of the sound generator 1 is used or a reflecting surface, or the sound generator 1 can be used as a sound receiver. In that case, a moment that an electric signal is applied to the sound generator to be operated is considered as an original measuring point. In that case, a delay time occurs due to an electronic circuit property, which is used to the outcome of the transit time measurement. Therefore, the delay time must be found out in advance and deleted from the measuring result. But, if the delay time is not constant, a measuring error of the sonic pulse transit time occurs. On the contrary, using the microphones 3 and 4 is free from the influence over the delay time.

[0059] As l₁ and l₂ is the interval exactly measured in advance, the sound velocities C₁ and C₂ is measured as follows: $\begin{matrix} {{C_{1} = \frac{l_{1}}{t_{1}}};{C_{2} = \frac{2l_{2}}{t_{2}}}} & (10) \end{matrix}$

[0060] But, the sound velocity C_(L) in the interval L cannot be measured, directly, because the interval L is a parameter changed according to the changing of the water level. It is assumed that the sound velocity C_(L) is as follows: $\begin{matrix} {C_{L}^{\prime} = {{\frac{1}{2}\left( {C_{1} + C_{2}} \right)} = {\frac{1}{2}\left( {\frac{l_{1}}{t_{1}} + \frac{2l_{2}}{t_{2}}} \right)}}} & (11) \end{matrix}$

[0061] Of course, a sound velocity C′_(L) obtained by the expression (11) may not be corresponded to the sound velocity C_(L) in the interval L. as the sound velocity C′_(L) is calculated, a distance L′ between the position of the microphone 3 and the water surface is as follows: $\begin{matrix} {L^{\prime} = {\frac{t_{L}C_{L}^{\prime}}{2} = {\frac{t_{L}}{2} \times \frac{1}{2}\left( {\frac{l_{1}}{t_{1}} + \frac{l_{2}}{t_{2}}} \right)}}} & (12) \end{matrix}$

[0062] Therefore, an error of the distance L′ is as follows: $\begin{matrix} {{\delta_{L} = {{\frac{L^{\prime}}{L} - 1} = {\frac{C_{L}^{\prime}}{C_{L}} - 1}}}{L = {\frac{t_{L}}{2}C_{L}}}} & (13) \end{matrix}$

[0063] Wherein,

[0064] Multiplying the expression (13) by 100 represents a relative error δ_(L). The relative error δ_(L) is analyzed as follows:

[0065] In case that the relative error is analyzed even in the prior arts such as the patents as mentioned above suggested, it is assumed that the air temperature in the wave-guide tube is changed in a linear as shown in FIG. 5. Under the condition, the relative error δ_(L) is analyzed as follows:

[0066] Considering that T₀ and T₂ each is an air temperature in the position of the microphones 3 and 4, T_(w) is a temperature of the water surface and T₃ is a temperature on the reflecting surface of the sound reflecting cylinder 6, average temperatures T_(l1) and T_(l2) in each of the interval l₁, l₂ and L are calculated as follows: $\begin{matrix} {T_{l_{1}} = {\frac{T_{0} + T_{2}}{2} = {\frac{T_{0} + T_{0} - {\frac{\Delta \quad T}{L}l_{1}}}{2} = {T_{0} - {\frac{\Delta \quad T}{2L}l_{1}}}}}} & (14) \\ {T_{l_{2}} = {\frac{T_{w} + T_{w} + {\frac{\Delta \quad T}{L}l_{2}}}{2} = {T_{w} + {\frac{\Delta \quad T}{2L}l_{2}}}}} & (15) \\ {T_{L} = \frac{T_{0} + T_{w}}{2}} & (16) \end{matrix}$

[0067] Wherein, ΔT=T₀−T_(W)

[0068] An expression between the sound velocity and the air temperature related to the average temperature is as follows:

C=C ₀+0.6τ° C.  (17)

[0069] C₁, C₂ and C_(L) are obtained by being substituted into the expression (17), wherein C₀=331.6 m/s when T=0° C., and a ratio of C′_(L) and C_(L) is obtained as follows: $\frac{C_{L}^{\prime}}{C_{L}} = {\frac{\frac{1}{2}\left( {C_{1} + C_{2}} \right)}{C_{L}} = \frac{0.5 \cdot {C_{0}\left\lbrack {1 + {\beta \left( {T_{0} - {\frac{\Delta \quad T}{2L}l_{1}}} \right)} + 1 + T_{L} + {\beta \left( {T_{w} + {\frac{\Delta \quad T}{2L}l_{2}}} \right)}} \right\rbrack}}{C_{0}\left\lbrack {1 + {\beta \left( \frac{T_{0} + T_{W}}{2} \right)}} \right\rbrack}}$

${Wherein},{\beta = {\frac{\alpha}{C_{0}} = {1.81 \times 10^{- 3}}}}$ ${{As}\quad a\quad {result}},{{\frac{C_{L}^{\prime}}{C_{L}}{is}\quad {{follows}:\frac{C_{L}^{\prime}}{C_{L}}}} = \frac{1 + {0.5{\beta \left\lbrack {T_{0} + T_{W} + {\frac{\Delta \quad T}{2L}\left( {l_{2} - l_{1}} \right)}} \right\rbrack}}}{1 + {0.5{\beta \left( {T_{0} + T_{W}} \right)}}}}$

[0070] Accordingly, the relative error δ_(L) is as follows: $\begin{matrix} {\delta_{L^{\prime}} = {\frac{1 + {0.5{\beta \left\lbrack {T_{0} + T_{W} + {\frac{\Delta \quad T}{2L}\left( {l_{2} - l_{1}} \right)}} \right\rbrack}}}{1 + {0.5{\beta \left( {T_{0} + T_{W}} \right)}}} - 1}} & (18) \end{matrix}$

[0071] In the expression (18), assuming that l₁=l₂=l, δ_(L′)=0.

[0072] Like this, in case that l₁=l₂=l in the wave-guide tube similar to that of a prior art, δ_(L′)=0

[0073] Herein, it is noted that the invention requires only two microphones 3 and 4 operated in air unlike the convention system including a number of microphones submerged in water and positioned in air, and significantly reduces the water level measuring error according to the change of the sound velocity in the wave-guide tube. The reasons are as follows:

[0074] If the air temperature at the upper of the wave-guide tube is higher than that at the lower of the wave-guide tube, the expression (11) is derived as follows: $\begin{matrix} {C_{L}^{\prime} = {{\frac{1}{2}\left( {C_{1} + C_{2}} \right)} = {{\frac{1}{2}\left( {C_{L} + {\Delta \quad C_{1}} + C_{L} - {\Delta \quad C_{2}}} \right)} = {C_{L} + \frac{{\Delta \quad C_{1}} - {\Delta \quad C_{2}}}{2}}}}} & (19) \end{matrix}$

[0075] Where, C_(L) is a sound velocity in the interval L. If ΔC₁=ΔC₂, C′_(L)=C_(L). On the contrary, if the air temperature at the upper of the wave-guide tube is lower than that at the lower of the wave-guide tube, C′_(L) is derived as follows: $C_{L}^{\prime} = {{\frac{1}{2}\left( {C_{L} - {\Delta \quad C_{1}} + C_{L} + {\Delta \quad C_{2}}} \right)} = {C_{L} + \frac{{\Delta \quad C_{2}} - {\Delta \quad C_{1}}}{2}}}$

[0076] (ΔC₁−ΔC₂) or (ΔC₂−ΔC₁) is always smaller than ΔC₁ or ΔC₂, one half of which is a measuring error of the sound velocity in the interval L.

[0077] But, under the condition of l₁=l₂=l, the case of ΔC₁=ΔC₂ never occurs. Therefore, δ_(L) cannot be equal to 0. The cause is as follows:

[0078] The average temperature T_(L) in the interval L can be calculated based on the expression (16). Because the temperature distribution curse does not appear to be a perfect linear, but can be represented as a linear corresponded to the average temperature in the interval L. Nevertheless, the temperature distribution state in the upper of the wave-guide tube on which the microphones 3 and 4 are mounted, for example the interval l₁ does not correspond to that nearby the water surface that the sound reflecting cylinder is mounted in, for example the interval l₂.

[0079] For example, when an ambient air temperature is higher than a water temperature in summer, the hot air in the wave-guide tube rises upward, so the temperature difference between the microphones 3 and 4 is relatively greater. In other words, in the interval l₁, the gradient degree of the temperature changing is higher. But, the wave-guide tube is “cooled” by the water near the water surface, the gradient degree of the temperature changing in the interval l₂ is less over the upper of the wave-guide tube.

[0080] On the contrary, if the ambient air temperature is too far lower than the water one, the gradient degree of the temperature changing in the interval l₂ becomes larger than that on the upper of the wave-guide tube. The temperature distribution state dividing the interval L into three is illustrated in FIG. 6.

[0081] Referring to FIG. 6, the dotted line is a straight line showing an average temperature gradient degree from a temperature T₀ at the mounting position of the microphone 3 to a one T_(W) at the water surface. The average temperature gradient is as follows: ${\tan \quad \alpha} = \frac{T_{0} - T_{W}}{L}$

[0082] The temperature changing gradients in the intervals I and II are as follows: ${\frac{3\left( {T_{0} - T_{1}} \right)}{L} = {\frac{3}{L}\Delta \quad T_{1}}};{\frac{3\left( {T_{\Pi} - T_{W}} \right)}{L} = {\frac{3}{L}\Delta \quad T_{III}}}$

[0083] Therefore, the average temperatures T_(l1), l_(l2) and the sound velocities C₁, C₂ in the intervals l₁ and l₂ are as follows: $T_{l_{1}} = {{\frac{1}{2}\left( {T_{0} + T_{0} - {\frac{3}{L}\Delta \quad {T_{1} \cdot l_{1}}}} \right)} = {T_{0} - {\frac{3}{2L}\Delta \quad {T_{1} \cdot l_{1}}}}}$ $T_{l_{2}} = {{\frac{1}{2}\left( {T_{W} + T_{W} + {\frac{3}{L}\Delta \quad {T_{III} \cdot l_{2}}}} \right)} = {T_{W} + {\frac{3}{2L}\Delta \quad {T_{III} \cdot l_{2}}}}}$

[0084] Accordingly, $\begin{matrix} {C_{1} = {C_{0}\left\lbrack {1 + {\beta \left( {T_{0} - {\frac{3}{2L}\Delta \quad {T_{1} \cdot l_{2}}}} \right)}} \right\rbrack}} & (20) \\ {C_{2} = {C_{0}\left\lbrack {1 + {\beta \left( {T_{W} + {\frac{3}{2L}\Delta \quad {T_{III} \cdot l_{2}}}} \right)}} \right\rbrack}} & (21) \end{matrix}$

[0085] at that case, C′_(L) is as follows: $\begin{matrix} {C_{L}^{\prime} = {{\frac{1}{2}\left( {C_{1} + C_{2}} \right)} = {C_{0}\left\{ {1 + {0.5{\beta \left\lbrack {T_{0} + T_{W} + {\frac{3}{2L}\left( {{\Delta \quad T_{III}l_{2}} - {\Delta \quad {T_{1} \cdot l_{1}}}} \right)}} \right\rbrack}}} \right\}}}} & (22) \end{matrix}$

[0086] The substitution of the expression (22) into the expression (13) of the relative error δ_(L) is as follows $\begin{matrix} {\delta_{L} = {{\frac{C_{L}^{\prime}}{C_{L}} - 1} = {\frac{1 + {0.5{\beta \left\lbrack {T_{0} + T_{W} + {\frac{3}{2L}\left( {{\Delta \quad T_{III}l_{2}} - {\Delta \quad {T_{1} \cdot l_{1}}}} \right)}} \right\rbrack}}}{1 + {0.5{\beta \left( {T_{0} + T_{W}} \right)}}} - 1}}} & (23) \end{matrix}$

[0087] In the expression (23), the establishment of δ_(L)→0 must meet the condition as follows:

ΔT _(III) l ₂ =ΔT ₁ l ₁  (24)

[0088] $\begin{matrix} {\frac{l_{1}}{l_{2}} = \frac{\Delta \quad T_{III}}{\Delta \quad T_{1}}} & (25) \end{matrix}$

[0089] Accordingly, if the relationship of the expression (25) is secured by adjusting the intervals l₁ and l₂ according to the temperature changing differences ΔT₁ and ΔT_(III) in the upper and lower of the wave-guide tube, the water level measuring error δ_(L) due to the measuring error of the sound velocity C_(L) in the interval L→0. If the relationship of the expression (25) is secured by adjusting the interval l₂, it is difficult to adjust the interval l₂ that is a length of the sound reflecting rode positioned at the lower of the wave-guide tube. For it, it is convenient to adjust the interval l₁ between the microphones 3 and 4 in order to meet the condition as follows: $\begin{matrix} {l_{1} = {\frac{\Delta \quad T_{III}}{\Delta \quad T_{1}} \cdot l_{2}}} & (26) \end{matrix}$

[0090] In order to obtain the interval l₁ by the expression (26), it is not necessary to measure the temperature changing differences ΔT₁ and ΔT_(III). The interval l₁ is adjusted as follows; under the worst condition, for example the condition that T₀ is the greatest and T_(W) is the lowest, for example T₀ is 40° C. and T_(W) is 22° C., the interval L between the microphone 3 and the water surface is accurately measured using a precision rule, and the interval l₁ is adjusted so that L=L′ through the comparison of the value L′ measured by the sonic water level measuring system with the interval L. On the contrary, under the condition that the ambient air temperature is too far lower than the water one, for example T₀ is −10° C. and T_(W)≈0° C., the interval l₁ is adjusted so that L=L′.

[0091] Therefore, if the interval l₁ is adjusted when |T₀−T_(W)| becomes the greatest, the measuring error of the interval L is reduced unless |T₀−T_(W)| exceeds the greatest value. Keeping up with this, the sonic water level measuring system is preferably corrected a few times in a year, for example every seasonal variation. The correction is performed when the water level is required to be very precisely measured. If the water level measuring allowance error Δ_(L)≈±10 mm, it is no matter to keep at l₁=l₂=l=const. In that case, the expression (23) is derived as follows: $\begin{matrix} {\delta_{L} = \frac{1 + {0.5{\beta \left\lbrack {T_{0} + T_{W} + {\frac{3l}{2L}\left( {{\Delta \quad T_{III}} - {\Delta \quad T_{1}}} \right)}} \right\rbrack}}}{1 + {0.5{\beta \left( {T_{0} + T_{W}} \right)}}}} & (27) \end{matrix}$

[0092] Table 1 presents an absolute value of the relative error δ_(L) followed by the changing of absolute values of $\frac{l}{L}$

[0093] and ΔT_(III)−ΔT₁. TABLE 1 δ_(L) = f(Δ T_(I); l/L) % l/L | ΔT_(m) − ΔT_(I) | ° C. 0.01 0.02 0.04 0.06 0.1 2.0 2.57 · 10⁻³ 5.14 · 10⁻³ 1.02 · 10⁻³ 1.54 · 10⁻³ 2.57 · 10⁻³ 4.0 5.14 · 10⁻³ 9.60 · 10⁻³ 2.05 · 10⁻³ 3.08 · 10⁻³ 5.14 · 10⁻³ 6.0 7.71 · 10⁻³ 1.54 · 10⁻³ 3.08 · 10⁻³ 4.62 · 10⁻³ 7.71 · 10⁻³

[0094] On the other hand, there seldom happens the situation of |ΔT_(III)−ΔT_(I)|=4° C. in site that the sonic water level measuring system is mounted. Assuming that the temperature difference of T₀−T_(W)=40° C. −22° C.=18° C. the temperature difference of ΔT_(III)−ΔT₁ in an interval of one-third between the upper and lower of the wave-guide tube=6° C. Herein, |ΔT_(III)−ΔT₁|=4° C., when T₁=10° C. and ΔT_(III)=6° C. If when ΔT₁=−10° C. and ΔT_(III)=2° C. in a cold weather, the average temperature difference T₀−T_(W)=−8° C., so the temperature difference in the one-third interval of L is around 2.66° C. Under this condition, ΔT₁ must have been −6° C. and ΔT_(III) must have been 2° C., so that |ΔT_(III)−ΔT_(I)|=4° C. In other words, under the worst condition, when |ΔT_(III)−ΔT_(I)|=4° C., assuming that L=10 m or L=50 m and l=1 m, the absolute error _(L) (mm) of the measurement of the interval L is calculated as follows:

[0095] When L=10 m, $\frac{l}{L}$

[0096] =0.1 and δ_(L)=5.14·10⁻²%=5.1410⁻⁴

Δ_(L)=10000 mm·5.14·10⁻⁴=5.14 mm

[0097] When L=50 m, $\frac{l}{L}$

[0098] =0.02 and δ_(L)=2.6·10⁻³%=2.610⁻⁵

Δ_(L)=50000·2.6·1−10⁻⁵=1.3 mm

[0099] Herein, it is contradictory that Δ_(L)=1.3 mm at time that L of 50 m is smaller than Δ_(L)=of 5.14 mm at time that L=10 m. The reason is because it assumes that T₀=40° C. and T_(W)=22° C. As a matter of fact, shorter the interval L is, less the temperature difference of T₀ and T_(W) is |ΔT_(III)−ΔT_(I)| becomes small more and more. And the experiment in site causes a larger error than one of L calculated as described above. The reason results from two causes.

[0100] First, it assume that the average temperature $T_{L} = \frac{T_{0} + T_{W}}{2}$

[0101] in the expression (24), but the average temperature $T_{L} = {\frac{T_{0} + T_{W}}{2} \pm \Delta_{T}}$

[0102] in fact according to the season, in which Δ_(T) is an error under the assumption that the air temperature in the wave-guide tube is changed in a linear constant gradient. Δ_(T) is usually subject to be smaller and doesn't exceed 1° C.

[0103] Nevertheless, when L=50 m, $\frac{l}{L}$

[0104] =0.02 and |ΔT_(III)−ΔT_(I)|=4°) C., δ_(L) is as follows:

δ_(L)=1.02·10⁻⁴>9.6·10⁻⁵=0.96·10⁻⁴

[0105] That is, the error is increased a little.

[0106] Secondary cause is because the measuring errors of the sonic pulse transit time t₁, t₂ and t_(L) are added. But, due to it, the water level measuring error can be effectively compensated. The compensating method is as follows:

[0107] In a place such as a room where the temperature difference scarcely exists, for example T₀≈T_(W), the wave-guide tube of the sonic water level measuring system is mounted, and the water temperature T_(W) in the lower of the wave-guide tube is subject to a room temperature. In that case, the measuring error of the sound velocity C_(L) doesn't occur because C₁=C₂=C_(L). T₀=40° C. and T_(W)=22° C. Only the measuring error of the interval L occurs due to the sonic pulse transit time measuring error.

[0108] Like this, the interval l₁ is adjusted to compensate for the water level measuring error due to the sonic pulse transit time measuring error.

[0109] The length l₂ of the sound reflecting rod is selected as follows: it is differentiated between the sonic pulses reflected on the sound reflecting rod and on the water surface without disturbing with respect to each other. The time interval t₂ between the two sonic pulses is as follows: $\begin{matrix} {t_{2} = \frac{2l_{2}}{C_{2}}} & (28) \end{matrix}$

[0110] But, referring to FIG. 2, the time interval until the sonic pulse is completely damped is as follows: $\begin{matrix} {\tau = \frac{N}{f}} & (29) \end{matrix}$

[0111] Wherein, N is the number of period until the sonic pulse is completely damped, and f is a frequency of the sonic pulse.

[0112] Therefore, t₂ is as follows: $t_{2} = {\frac{2l_{2}}{C_{2}} \geq \frac{N}{f}}$

[0113] For example, if N=5 and f=1000 Hz, ^(t) ^(₂) ^(≧5·10) ⁻³ ^(S). If the maximum sound velocity C₂=350 m/s in the interval l₂, l₂ is as follows: ${l_{2} \geq \frac{N \cdot C_{2}}{2f}} = {\frac{5 \cdot 350}{2 \cdot 1000} = {0.875m}}$

[0114] Therefore, l₂ is selected to be around 1 m. But, if the interval l₁ between the microphones 3 and 4 is selected to be equal to l₂, the outputting signals of the microphones 3 and 4 are overlapped. The reason is because $t_{1} = {\frac{l_{1}}{C_{1}}.}$

[0115] The following expressions are established. ${t_{1} = {\frac{l_{1}}{C_{1}} \geq \frac{N}{f}}};$ ${l_{1} \geq \frac{NG}{f}} = {\frac{5 \cdot 350}{1000} = {{1.75m} \succ l_{2}}}$

[0116] In order that l₂≈l₂, the outputting signals of the microphones 3 and 4 must be inputted to their corresponding amplifier, not one amplifier, to be respectively amplified.

[0117] A sonic water level measuring system for realizing the sonic water level measuring method according to the invention will be described below.

[0118] Referring to FIG. 7, 1 is a sound generator, 2 is a wave-guide tube, 3 and 4 are a microphone, 6 is a sound reflecting cylinder, 7 is a float, 8 ₁ and 8 ₂ are a microphone rod for mounting the microphones 3 and 4 and 18 is a case for protecting the sound generator 1.

[0119] On the other hand, an electronic circuit is constituted as follow: 9 is an oscillator for operating the sound generator 1. 10 and 11 are amplifiers for amplifying the outputting signals of the microphones 3 and 4. 12 and 13 are zero-crossing circuits for catching a moment that the outputting signals of the microphones 3 and 4 pass through the zero potential at time of ending its first period. 14 is a microprocessor, 15 is a display, 16 is an outputting signal converter and 17 is a saw-tooth pulse generator. The saw-tooth generator 17 is controlled by the microprocessor 14 to begin to be operated from a moment that the sound generator 1 generates sonic pulses and then to be stopped at the moment of receiving sixth pulse amplified passing through the amplifiers 10 and 11. The microphone 3 is fixed to the microphone rod 8 ₂ on the position spaced in over an approximate inner diameter D of the wave-guide tube 2 from the bottom surface of the case 18, where a planar wave is shaped. Under the condition that the air temperature in the upper of the wave-guide tube 2 is higher, it is preferable to pull down the microphone 3 a lot. The microphone 4 is fined to the lower end of the microphone rod 8 ₁, in which the microphone rod 8 ₁ is extended piercing through the case 18. The microphone rods 8 ₁ and 8 ₂ are in the form of a tube, through which a cable is passed.

[0120] The interval l₁ between the microphones 3 and 4 is adjusted by moving the microphone rod 8 ₁. The float 7 and the sound-reflecting cylinder 6 are connected to each other. The interval l₂ between the upper surface of the sound reflecting cylinder 6 and the water surface is set by the expression (20).

[0121] The wave-guide tube 2 is made of PVC, SUS, Metal Pipe, etc. according to the mounting condition and the length thereof, its inner diameter D being selected to be smaller than a wave length of the sonic pulse, so that the transit condition of the planar wave is secured.

[0122] The sonic water level measuring system is operated as follows:

[0123] The oscillator 9 is operated at a constant period according to the programming inputted into the microprocessor 14. The sound generator 1 receives the outputting signals of the oscillator 9 to generate sonic pulses. The sonic pulses are transited along the wave-guide tube 2 toward the water surface. At that time, the microphones 3 and 4 receive incident waves in order. The incident waves are reflected on the sound-reflecting rod 6 and the water surface and then transited toward the upper of the wave-guide tube 2. The microphones 3 and 4 again receive the reflected waves in order contrary to the receiving of the incident wave.

[0124] The microphones 3 and 4 are respectively connected to the amplifiers 10 and 11 to amplify the outputting signals thereof. The zero crossing circuits 12 and 13 are connected to the amplifiers 12 and 13, respectively, to form a single pulse as shown in FIGS. 4B and 4C, when the amplified signals reaches a zero potential at the falling edge every period. The microprocessor 14 receives the outputting signals from the zero-crossing circuits 12 and 13 to measure transit times of t₁, t₂ and t_(L) and compute the distance L from the position of the microphone 3 to the water surface based on the expression (12).

[0125] The display 15 represents the result of computing the distance L by the microprocessor 14. The converter 16 is connected to the microprocessor 14 to convert the outputting signals from the microprocessor 14 into a signal such as a code signal of RS232C having the outputting of 4˜20 mA that are necessary for a telemeter, an automatic water level recorder, etc.

[0126] When the sonic incident wave is transited toward the microphones 3 and 4, the sound pressure is relatively larger, and the outputting voltages of the microphone 3 and 4 are relatively higher. But, when the signals on the sound reflecting cylinder 6 and the water surface reaches the microphones 3 and 4, their sound pressure gets smaller by a larger amount, and the outputting voltages of the microphones 3 and 4 becomes more lower. Due to it, the amplifiers 10 and 11 are controlled to output the same voltage independent of receiving the incident wave or the reflected wave in order to operate the zero-crossing circuits 12 and 13, normally. To it, the saw-tooth pulse generator 17 adjusts the amplification factor of the amplifiers 10 and 11 as shown in FIG. 8.

[0127]FIG. 8A shows the outputting voltage V_(3;4) of the microphones 3 and 4, FIG. 8B shows the outputting voltage V₁₇ of the saw-tooth pulse generator 17, FIG. 8C shows the change of a amplification factor V_(10;11) of the amplifiers 10 and 12 and FIG. 8D shows the outputting voltage V_(10;11) of the amplifiers 10 and 11. Herein, the amplifiers 10 and 11 permits their amplification factors to be adjusted by the voltage applied thereto.

[0128] The saw-tooth pulse generator 17 begins being operated by a signal that the microprocessor 14 operates the oscillator 9 and is subject to being stopped at a moment that the zero-crossing circuits 12 and 13 outputs the sixth pulse.

[0129] For example, if L=20 m and C_(L)=350 m/s, the operating time t_(L) of the saw-tooth pulse generator 17 is as follows:

t _(L)=2L/C _(L)=40/350=0.114 s.

[0130]FIG. 9 shows the mounting of the microphones 3 and 4, the microphone 3 is mounted below the bottom surface of the case 8 by means of the microphone rod 8 ₂. The microphone 4 is fixed at the lower end of the microphone rod 8 ₁. The microphone rod 8 ₁ is extended passing through the case 18 and has a scale thereon. A plurality of holes are perforated near the upper end of the wave-guide tube 2 to discharge hot air there from outside in summer. The interval l₁ between the microphones 3 and 4 is adjusted and fixed by moving the microphone rod 8 ₁. In FIG. 9, arrow lines represent a cable each of which is connected to the amplifiers 10 and 11.

[0131] Referring to FIG. 10, the float 7 is integrated with the sound-reflecting cylinder 6 ₁. The sound-reflecting cylinder 6 includes two cylinders 6 ₁ and 6 ₂. The cylinder 6 ₁ is closed at the upper end, and the cylinder 6 ₂ is fitted into the cylinder 6 ₁. The cylinder 6 ₁ is configured to adjust the interval l₂ between the upper surface thereof and the water surface. The float 7 has a weight portion at the lower and a closed space at the upper to adjust a buoyant force. It is preferable to submerge the float 7 in the water by 1 cm. In order to adjust the depth to be submerged, the float 7 is provided with a valve to supply distilled water there into. The float 7 also includes at least one ring 20 having a semicircle section. The ring 20 rocks the float 7 upon the going up and down of the float 7 together with the water level changing. Therefore, the ring 20 limits the rocking amplitude and permits the float to be freely moved up and down. For it, the ring 20 is subject to minimize the friction with the inner wall of the wave-guide tube 2. The ring 20 is made of Teflon, etc. that has a lower friction coefficient and the rustproof property. More preferably, the float 7 is made of Fluoric-plastic containing Fluorine for preventing the propagation of water bacteria. The inner diameter of the ring 20 is made smaller than that of the wave-guide tube 2 by 3%. If the wave-guide tube 2 is mounted on a slant for measuring the water level, for example the water level H is measured in a manner to calculate a height h from an original water level measuring point to the water surface using a expression h=L×sin α, in which α is a gradient angle. The water level H=H₀−h, wherein H₀ is a height above sea level of the original water level measuring point, and the upper surface of the float 7 is projected above the water surface with its buoyant force being adjusted.

[0132]FIG. 11 shows a state that the float 7 is floated over the water surface. At that time, the interval L that is not measured by the sonic wave is as follows: ${\Delta \quad L} = {{\frac{D}{2}\tan \quad \alpha} = {const}}$

[0133] Wherein, D is an inner diameter of the wave-guide tube 2.

[0134] Therefore, the height h=(L+ΔL) sin α.

[0135] The diameter d of the sound-reflecting cylinder 6 is selected as follows: based on the experiment, when the length l₂ of the sound-reflecting cylinder 6 is around 1 m, if the ratio of an area S_(d) of the reflecting surface and an area S_(W) of the water surface is secured to be 1:3, the intensities of the reflecting waves are similar to each another. $d = {\frac{D}{\sqrt{4}} \approx {0.5D}}$

[0136] is selected, wherein D is an inner diameter of the wave-guide tube. $\left( {{\frac{S_{W}}{S_{d}} = {\frac{\left( {D^{2} - d^{2}} \right)}{d^{2}} = {{\frac{D^{2}}{d^{2}} - 1} = 3}}},{{\therefore d} = {\frac{D}{\sqrt{4}} = {0.5D}}}} \right)$

[0137] In that case, the float 7 is floated on the water, adjust the submerging state thereof and the length l₂ of the sound-reflecting cylinder 6 and then thrown into the wave-guide tube 2.

[0138] According to the invention, the sonic water level measuring system is too much simple over the conventional water level meter, need not a number of sound receivers and secure the high accuracy of the water level measurement independent of the changing of the ambient weather condition. Furthermore, if it is intended to measure the water level, more accurately, the interval l₁ is simply adjusted according to a seasonal variation. 

What is claimed is:
 1. A sonic water level measuring method for fixing an original measuring point on the upper of a wave-guide tube in the form of a pipe, measuring times that it takes for a sonic pulse to be transited through air medium in the wave-guide tube toward a water surface and transited back to the original measuring point after being reflected on the water surface and computing a distance from the original measuring point to the water surface comprising steps of: arranging two microphones constituted as a sound receiver to be spaced away in an interval l₁ from each other below a sonic pulse generator on the upper of the wave-guide tube; submerging a float integrated with a sound reflecting cylinder to keep an interval l₂ between the water surface and the reflecting surface of the sound-reflecting cylinder; measuring times t₁, t₂ and t_(L) that it takes for the sonic pulse to transit the intervals l₁ and l₂ and to go up and down a distance L from a first microphone to the water surface, respectively; calculating the distance L from the first microphone to the water surface as follows: $L = {{\frac{t_{L}}{2}\left( {\frac{l_{1}}{t_{1}} + \frac{l_{2}}{t_{2}}} \right)} = {\frac{1}{4}{t_{L} \cdot \left( {\frac{l_{1}}{t_{1}} + \frac{l_{2}}{t_{2}}} \right)}}}$

Only, l₂ is selected as follows: $l_{2} \geq \frac{{NC}_{2}}{2f}$

wherein, f is a sonic pulse frequency, N is the number of period until the sonic pulse is completely damped and C₂ is a maximum sound velocity that can be expected in the interval l₂.
 2. The sonic water level measuring method as claimed in claim 1, in which: the interval l₁ between the two microphones is adjusted to minimize the water level error as follows: $l_{1} = {l_{2}\frac{\Delta \quad T_{III}}{\Delta \quad T_{I}}}$

wherein, ΔT₁=T₀−T₁ and ΔT_(III)=T_(II)−T_(W), T₀ and T_(W) are an air temperature near the microphone disposed at the original measuring point of L on the upper of the wave-guide tube and a temperature of the water surface, and T_(I) and T_(II) are air temperatures on the upper and lower of L in the wave-guide tube.
 3. The sonic water level measuring method as claimed in claim 1 or 2, in which: if |ΔT_(III)−ΔT_(I)|<6° C. and the water level measuring allowance error is ±10 mm, the water level is computed using l₁=l₂=const. independent of a water level measuring range.
 4. A sonic water level measuring system including a sound generator, a wave-guide tube, a sound receiver mounted in the wave-guide tube, an amplifier and a microprocessor comprising: a first microphone mounted in the wave-guide tube to keep an interval more than an inner diameter of the wave-guide tube from the sound generator mounted on the upper of the wave-guide tube; a second microphone mounted on the lower end of a microphone rod, which is movable up and down, to keep an interval l₁ downward along the wave-guide tube from the first microphone; a float floating in water and integrated with a sound-reflecting cylinder which is extended by the interval l₂ from the water surface in the wave-guide tube; a sound generator electrically connected to an oscillator for operating it; two amplifiers connected to each of the first and second microphones; two zero-crossing circuits connected to each of the outputs of the two amplifiers; and, the microprocessors connected to the outputs of the two zero-crossing circuits; a saw-tooth generator connected to the two amplifiers to increase the amplification factor thereof, which is controlled by the microprocessor to be operated at a moment that the sonic pulse is generated and stopped at a moment that the zero-crossing circuit generates 6^(th) pulse; in which the microprocessor has a system programming previously inputted there into to operate the oscillator at a constant period, memorizes the intervals l₁ and l₂ at a memory, measures times that it takes for the sonic pulse to transit the intervals l₁ and l₂ and to go up and down a distance L from a first microphone to the water surface, respectively, and computes the value of the distance L, and if the wave-guide tube is vertically mounted, the upper surface of the float is submerged in water by 1 cm, and if the wave-guide tube is slantingly mounted, the upper surface of the float is projected above the water surface to its buoyant force being adjusted.
 5. The sonic water level measuring system as claimed in claim 4, in which: a diameter d of the reflecting surface of the sound-reflecting cylinder is selected to be a half of the inner diameter D of the wave-guide tube, the float includes a ring having a semicircle section and made of Teflon and the outer diameter of the ring is smaller that the inner diameter D of the wave-guide tube by 3%.
 6. The sonic water level measuring system as claimed in claim 1 or 4, in which: the water level measuring system further comprises a correcting method including steps of mounting the sonic water level measuring system in a room that a temperature difference between the upper and lower of the wave-guide tube scarcely exists; measuring the distance L from the original water level measuring point to the water surface by a precise length measuring means; and, adjusting the interval l₁ between the two microphones to correspond a value of L′ measured by a sonic water level meter to the value of L in order to compensate for the water level measuring error that occurs according to a sonic pulse transit time measuring error. 